Method for forming a metal layer in multiple steps

ABSTRACT

A method for forming a metal layer having a predetermined thickness on an underlying material is disclosed. According to the method, the underlying material is electroplated to form the metal layer having a fraction of the predetermined thickness thereon. The step of electroplating is interrupted for a predetermined period of time. The step of electroplating is then resumed to form the metal layer having the predetermined thickness on the underlying material, thereby improving planarity of the metal layer.

BACKGROUND

The present invention relates generally to semiconductor processingtechnology, and more particularly to a method for forming a metal layerin multiple steps.

Semiconductor integrated circuits (ICs) have many levels of patternedmetal layers. Different levels of metal layers can be connected byinterconnection structures, such as vias and cross-over trenches thatcontain inlaid conductive materials. The process of forming the metallayers is usually referred to as metallization. Copper is usuallyselected as the material for forming the metal layers, because of itssuperior electrical conductivity.

The copper layer is usually formed by an electroplating process. A thinseed layer is deposited on an underlying material, such as asemiconductor substrate or a dielectric layer. The underlying material,on which the seed layer is disposed, is placed in a chemicalelectroplating solution or a chemical reaction chamber. The seed layerthen grow into a thicker copper layer atop the underlying material. Thecopper layer is then patterned to form a desired conductive structure.

In certain applications, some very thick copper layers are needed. Thosethick copper layers often cause the problem of “hillock.” During theelectroplating process, each grain grows in an individual crystalorientation. As growth proceeds, grains fill the space between them andgenerally grow upward at similar rates. However, some grains find earlynucleation and have a head start in growth. Also, some grains grow at acrystal orientation that promotes a faster growth rate. Such grains thatgrow substantially taller than their neighbors are identified as“hillocks.” As the thickness of the copper layer increases, the problemof “hillock” becomes more serious. When the thickness of the copperlayer is under 13K angstroms, the grain size of copper is relativelysmall, and does not cause serious problems to the copper layer. However,when the thickness of the copper layer is above 40K angstroms, thecopper grain may become a “hillock,” and cause serious problems to thecopper layer.

The “hillocks” may cause certain drawbacks. One drawback is theirregular etching rates over the surface of the copper layer due to the“hillocks.” Another drawback is that the “hillocks” may increase thepossibility of bumping failures when using the copper layer as a bondingpad during an IC packaging process.

Therefore, desirable in the art of semiconductor processing technologyis a method for forming a metal layer without suffering from the“hillock” problems.

SUMMARY

The present invention discloses a method for forming a metal layerhaving a predetermined thickness on an underlying material. In oneembodiment of the invention, the underlying material is electroplated toform the metal layer having a fraction of the predetermined thicknessthereon. The step of electroplating is interrupted for a predeterminedperiod of time. The step of electroplating is then resumed to form themetal layer having the predetermined thickness on the underlyingmaterial, thereby improving planarity of the metal layer.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a semiconductor structurehaving a metal layer with “hillocks” generated during a conventionalelectroplating process.

FIG. 2 illustrates a cross-sectional view of a semiconductor structurehaving a metal layer of a fraction of a predetermined thickness inaccordance with one embodiment of the present invention.

FIG. 3 illustrates a cross-sectional view of a semiconductor structurehaving a metal layer of the predetermined thickness in accordance withone embodiment of the present invention.

DESCRIPTION

FIG. 1 illustrates a cross-sectional view of a semiconductor structure100 having a metal layer 106 formed thereon by a conventionalelectroplating process. The semiconductor structure 100 includes anunderlying material 102, which can be a semiconductor substrate or adielectric layer. A seed layer 104 is formed on the underlying material102. During an electroplating process, the underlying material 102together with the seed layer 104 is placed in a chemical electroplatingsolution or a chemical reaction chamber. The seed layer 104 then growsinto a metal layer 106 of a certain thickness. As discussed above, whenthe thickness of the metal layer 106 is large enough, a plurality ofmetal grains 108 will extend substantially beyond the surface of themetal layer 106 and become undesirable “hillocks.” This causes problems,such as uneven etching rates over the surface of metal layer 106, andhigher possibility of bumping failure when using the metal layer 106 asa bounding pad during an IC packaging process.

FIGS. 2 and 3 present a process flow of forming a metal layer withreduced grain size in accordance with one embodiment of the presentinvention. FIG. 2 illustrates a cross-sectional view of a semiconductorstructure 200. The semiconductor structure 200 includes an underlyingmaterial 202, which can be a semiconductor substrate or a dielectriclayer. A seed layer 204 is formed on the underlying material 202. Duringan electroplating process, the underlying material 202 together with theseed layer 204 is placed in a chemical electroplating solution or achemical reaction chamber. The seed layer 204 then grows into a metallayer 206. Before the metal layer 206 grows into its full predeterminedthickness, the electroplating process is interrupted for a predeterminedperiod of time.

In this embodiment, the metal layer 206 is made of copper. However,other conductive materials, such as aluminum, titanium, tantalum,cobalt, nickel, and an alloy thereof, can also be selected as thematerial for the metal layer 206. The predetermined time period ofinterruption can be any time period more than one second. Thepredetermined thickness is the thickness eventually the metal layer 206will have after the processing steps present by FIGS. 2 and 3 arecompleted. The thickness of the metal layer 206 is only a fraction ofthe predetermined thickness. For example, the predetermined thicknessmay be 40K angstroms, and the thickness of the metal layer 206 is only50 percent of the predetermined thickness. In other words, the thicknessof the metal layer 206 is about 20K angstroms, which is relatively thinas opposed to the whole predetermined thickness. As a result, the grains208 of the metal layer 206 would not become hillocks that make thesurface of metal layer 206 uneven.

During the interruption of the electroplating process, the semiconductorstructure 200 is removed from the chemical electroplating solution orthe chemical reaction chamber. This interrupts the continuous growth ofthe grains 208.

FIG. 3 illustrates a cross-sectional view of a semiconductor structure300, which is sued to explain the step of resuming the electroplatingprocess. After the interruption, the semiconductor structure 200 (seeFIG. 2) is reintroduced into the chemical electroplating solution or thechemical reaction chamber. The electroplating process is resumed and themetal layer 206 further grows into the full predetermined thickness withthe addition of the metal layer 206′. In this embodiment, thepredetermined thickness is in a range between 1 and 100K angstroms. Dueto the interruption, new nucleation sites are created. In most cases,plating new metal grains does not continue with the same crystalorientation or locations as in the original metal grains 208 (see FIG.2). The additional metal layer 206′ has a different grain structure thanthe originally grown metal layer 206. While grains 304 make theadditional metal layer 206′ uneven, they do not present a “hillock”problem. Similarly, while a rare grain 306 continues to grow from themetal layer 206, it does not present a significant “hillock” problem,either. As such, the planarity of the surface of the metal layer 206′ isimproved.

The advantage of this invention is that the totality of the irregularityof the originally grown metal layer and the irregularity of theadditional metal layer is less than that of the irregularity of a metallayer formed by the conventional one-step electroplating process. Thefull predetermined thickness of the metal layer may be divided into morethan two metal layers of fractional thickness. The steps of interruptingand resuming the electroplating process can be repeated for many timesto form a metal layer of a desirable thickness without generating“hillocks.” Thus, the present invention helps to provide the surface ofmetal layer with even etching rates. It also helps to reduce thepossibility of bumping failure when using the metal layer as a boundingpad during an IC packaging process.

The above illustration provides many different embodiments orembodiments for implementing different features of the invention.Specific embodiments of components and processes are described to helpclarify the invention. These are, of course, merely embodiments and arenot intended to limit the invention from that described in the claims.

Although the invention is illustrated and described herein as embodiedin one or more specific examples, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the scope of the invention, asset forth in the following claims.

1. A method for forming a metal layer having a predetermined thicknesson an underlying material comprising: electroplating the underlyingmaterial to form the metal layer having a fraction of the predeterminedthickness thereon; interrupting the electroplating for a predeterminedperiod of time; and resuming the electroplating to form the metal layerhaving the predetermined thickness on the underlying material, therebyimproving planarity of the metal layer.
 2. The method of claim 1 whereinthe metal layer is a copper layer.
 3. The method of claim 1 wherein thepredetermined period of time is at least one second.
 4. The method ofclaim 1 wherein the predetermined thickness is between 1 and 100Kangstroms.
 5. The method of claim 1 wherein the step of electroplatingfurther comprises placing the underlying material in a chemical solutionor a chemical reaction chamber.
 6. The method of claim 5 wherein thestep of interrupting further comprising removing the underlying materialfrom the chemical solution or the chemical reaction chamber.
 7. Themethod of claim 1 wherein the steps of interrupting and resuming arerepeated for a plurality of times, depending on the predeterminedthickness.
 8. A method for forming a metal layer having a predeterminedthickness on an underlying material comprising: placing the underlyingmaterial in a chemical solution or a chemical reaction chamber;electroplating the underlying material to form the metal layer having afraction of the predetermined thickness thereon; removing the underlyingmaterial from the chemical solution or the chemical reaction solutionfor a predetermined period of time; placing the underlying material inthe chemical solution or the chemical reaction chamber; and resuming theelectroplating to form the metal layer having the predeterminedthickness on the underlying material, thereby improving planarity of themetal layer.
 9. The method of claim 8 wherein the metal layer is acopper layer.
 10. The method of claim 8 wherein the predetermined periodof time is at least one second.
 11. The method of claim 8 wherein thepredetermined thickness is between 1 and 100K angstroms.
 12. The methodof claim 8 wherein the steps of removing, placing and resuming arerepeated for a plurality of times, depending on the predeterminedthickness.
 13. A method for forming a copper layer having apredetermined thickness on an underlying material comprising: placingthe underlying material in a chemical solution or a chemical reactionchamber; electroplating the underlying material to form the copper layerhaving a fraction of the predetermined thickness thereon; removing theunderlying material from the chemical solution or the chemical reactionsolution for a predetermined period of time; placing the underlyingmaterial back in the chemical solution or the chemical reaction chamber;and resuming the electroplating to form the copper layer having thepredetermined thickness on the underlying material, thereby improvingplanarity of the metal layer.
 14. The method of claim 13 wherein thepredetermined period of time is at least one second.
 15. The method ofclaim 13 wherein the predetermined thickness is between 1 and 100Kangstroms.
 16. The method of claim 13 wherein the steps of removing,placing and resuming are repeated for a plurality of times, depending onthe predetermined thickness.